92168.pdf

218 Colloid stability

Table 8.3 Values of Hamaker constants^105

Material AH (microscopic) AH (macroscopic)

1(T^20 J 10"

Water

Ionic crystals

Metals

Silica

Quartz

Hydrocarbons

Polystyrene

3.3- 6.4

15.8-41.8

7.6-15.9

50

11.0-18.6

4.6-10

6.2-16.8

3.0- 6.1

5.8-11.8

22.1

8.6

8.0- 8.8

6.3

5.6- 6.4

(a) (T)(T%, (T

V_x <»..../ V^

|b|

O© ©0

Figure 8,1

the interactions are particle-dispersion medium interactions, with

Hamaker constants A 13 and A 23. K particle 2 is brought close to

particle 1 (Figure 8.1b), dispersion medium must be displaced to the

position originally occupied by particle 2 and the above interactions

are replaced by particle-particle and dispersion medium-dispersion

medium interactions, with Hamaker constants A\ 2 and A 33. The

effective Hamaker constant is, therefore, given by

32 (8.11)

If the attractions between unlike phases is taken to be the geometric

mean of the attractions of each phase to itself (cf. equation 4.3) - i.e.

equation (8.11) becomes

^02 = (Aft - A£) (A£ - A&) (8.12)